A practical way to double the bit carrying capacity of an existing unidirectional fiber optic communication link is by the use of optical circulators. An optical circulator is a passive, non-reciprocal device which permits full duplex communication on a single fiber optic link. Thus, a typical fiber optic communication link operating on two fibers can be quickly and economically converted to a bi-directional, single fiber communication link by installing an optical circulator at each end of the link.
One of the major advantages of optical circulators over more traditional 3 dB couplers is that the loss penalty is much lower. Using a 3 dB coupler at each end of a fiber link, there is an insertion loss of at least 6 dB. For connections which operate near their detection limits, this additional 6 dB loss could make bi-directional communication impracticable.
In a real optical circulator insertion loss and cross-talk as well as simplicity and low cost are important considerations. Insertion loss is the difference in power between light launched into the optical circulator and the power that exits the device. Insertion loss is largely due to absorption of light and to coupling loss.
Prior art optical circulators are described in U.S. Pat. No. 4,650,289, issued to Kuwahara; U.S. Pat. No. 4,464,022, issued to Emkey; and in U.S. Pat. No. 4,859,014, issued to Schmitt et al. However, optical circulators made as described in these references either have high insertion loss and/or cross-talk, or are very complicated and costly. Therefore, a need exists for an optical circulator having lower insertion loss and cross-talk and simpler configuration than that found in present optical circulators.
One factor that contributes to lower insertion loss and cross-talk as well as simplicity in the optical circulator of the present invention than in prior optical circulators is the use of folded configuration with birefringent crystals and polarization spitting cubes.
A more recent prior art optical circulator that uses birefringent crystals in contact with other elements such as polarization rotators, is U.S. Pat. No. 5,204,771 issued Apr. 20, 1993 in the name of Koga. Although this invention appears to perform its intended function adequately, Koga's configuration results in a large device compared with the folded configuration of the present invention. Koga's configuration is limited in other ways. For example, there are limitations as to which ports can be circulated in a backwards direction. Koga's device allows backward control in a 4 port device however does not provide or allow backward signal propagation from port 3 to port 1 in a three port device.
Another still more recently issued U.S. Pat. No. 5,471,340 in the name of Cheng et al., issued Nov. 28, 1995 provides a simpler configuration than Koga. However, the cost of manufacturing this device is considerable due to the large size of the birefringent crystals required. Furthermore, this device is also limited in not being capable of performing a full circulating function.
It is therefore an object of the invention, to overcome many of the limitations of known prior art devices.
It is a further object of the invention to provide a compact optical circulator that is of a folded configuration.